[Reprinted  from  PAPER,  October  30,   1918] 


GUIDE  TO  THE  BLEACHING  OF  PULP  AND  PAPER 

Processes   for  the   Bleaching   of  Vegetable   Fibers   in   the 
Pulp  and  Paper  Mill 

By  JAMES   BEVERIDGE 
Author  of  The  Papermakers'  Pocket  Book 

|HE  bleaching  of  vegetable  fibers  for 
papermaking  has  become  in  recent 
years  not  only  an  important  but  a 
widespread  operation  on  the  Amer- 
ican Continent,  and  for  obvious  rea- 
sons. Pulpmakers  especially  are 
adopting  bleaching  methods  in  order 
to  enhance  the  value  of  their  product, 
thereby  reaping  an  additional  profit. 
It  is  therefore  important  that  the  different  methods 
of  bleaching  fibers,  whether  these  be  derived  from  the 
acid  or  alkaline  processes  of  manufacture,  should  be 
studied  carefully  to  assure  maximum  economy  not 
alone  in  the  bleaching  agent  used,  but,  also,  in  all  the 
factors  involved  in  the  operation  as  a  whole.  These 
factors  so  far  as  the  bleaching  operation  itself  is  con- 
cerned are  controlled  largely  by  the  character  and 
purity  of  the  fiber  treated.  Thus  in  the  case  of  linen 
and  cotton  rag  stock,  the  bleaching  process  is  reduced 
to  its  simplest  terms,  in  virtue  of  the  comparative 
freedom  of  such  stock  from  those  foreign  vegetable 
substances,  which  are  acted  upon  and  rendered  soluble 
by  the  oxidizing  or  bleaching  agent. 

In  the  case  of  wood  and  other  pulps  such  as  straw, 
esparto,  bamboo,  bagasse,  etc.,  the  presence  of  for- 
eign vegetable  matter — other  than  cellulose — in  some- 
what larger  quantity  necessitates  a  more  drastic  treat- 
ment, resulting  in  a  larger  consumption  of  bleaching 
agent  per  unit  of  pulp  treated  and  a  greater  propor- 
tionate loss  of  weight.  In  point  of  fact,  these  two 
factors — viz. :  the  consumption  of  bleaching  agent  and 
loss  of  weight — other  conditions  of  the  treatment  be- 
ing kept  normal  or  nearly  so — are  a  measure  of  the 
quality  or  purity  of  the  pulp  itself.  This  is  under- 
stood by  all  practical  pulpmakers  and  their  constant 
aim  is,  or  should  be,  so  to  adjust  their  manufacturing 
conditions  that  they  can  obtain  a  product  which  will 
conform  to  a  minimum  consumption  of  bleaching 
agent  and  minimum  loss  of  weight. 


445,160 


The  bleaching  properties  of  a  pulp  depend  upon 
the  process  by  which  it  is  prepared.  As  a  general  rule 
vegetable  fibers  prepared  by  the  acid  or  sulphite  pro- 
cess bleach  more  readily  than  those  prepared  by  either 
of  the  alkaline  methods,  that  is  by  either  the  "caustic 
soda"  or  "sulphate"  methods  .  In  all  cases,  no  matter 
what  process  is  in  use,  substances  other  than  cellulose, 
as  above  stated,  are  left  behind  with  the  fiber,  and  it 
is  these  substances  that  are  acted  on  by  the  bleaching 
agent  and  rendered  soluble.  It  may  be  stated  here 
that,  of  the  three  acid  solutions — viz. :  bisulphite  of 
lime,  magnesia  and  soda,  it  has  been  found  in  actual 
manufacturing  practice,  those  of  magnesia  or  soda 
yield  a  purer  cellulose  than  that  obtained  with  lime. 
And  no  doubt,  this  is  due  to  the  fact  that  the  sulphur 
salts  of  magnesia  and  soda  are  much  more  soluble 
than  those  of  lime. 

The  quality  of  the  water  used  for  washing  the  crude 
pulp  has  also  an  influence  on  the  bleaching  process, 
especially  if  the  water  contains  lime  in  any  form.  In 
the  sulphite  process,  the  lime  salts  precipitate  insoluble 
resin  soaps  on  the  surface  of  the  fiber,  which  absorb 
chlorine  in  proportion  to^  their  presence,  and  in  order 
to  avoid  such  precipitation  calcareous  water  previously 
heated  to  boiling,  or  otherwise  chemically  treated  for 
the  removal  of  the  lime  and  filtered,  is  frequently  used. 
A  similar  precipitation  of  lime  salts  takes  place  when 
calcareous  water  is  used  for  washing  the  pulp  prepared 
by  the  alkaline  process,  the  lime  itself  being  precip- 
itated either  in  the  form  of  carbonate  or  sulphate  by 
the  alkali  present,  both  of  which  cling  to  the  fiber  and 
carry  down  with  them  coloring  organic  matter,  ren- 
dering the  process  of  bleaching  even  more  difficult 
and  costly  than  is  the  case  with  sulphite  pulps. 

The  loss  of  weight  in  bleaching  pulp,  together  with 
the  cost,  depend  on  many  factors,  the  most  important 
of  which  may  be  enumerated  as  follows : 

i — The  raw  fibrous  plant  from  which  the  pulp  is 
prepared:  Spruce,  hemlock,  balsam,  pine  and  others, 
among  the  conifers;  poplar,  maple,  birch,  and  others 
of  the  broadleafed  trees;  straw,  esparto,  bamboo, 
bagasse,  etc.,  among  the  grasses  and  canes. 

2 — The  process  employed  for  manufacturing  the 
cellulose  or  fiber,  whether  this  be  acid  or  alkaline,  i.  e., 
soda  or  sulphite  process. 

3 — The  purity  of  the  pulp  obtained,  controlled  large^ 
ly  by  the  conditions  under  which  the  fiber  is  prepared, 
such  as  the  amount,  character  and  composition  of  the 
resolving  chemicals  used;  the  temperature  employed; 


and  the  time  given  to  complete  the  process,  together 
with  the  purity  of  the  water  used  for  washing. 

4 — The  dilution  of  the  pulp  with  water,  or  density 
of  the  stock,  during  the  bleaching  operation,  which  in- 
sures a  more  intimate  and  closer  contact  of  the  bleach- 
ing agent  with  the  fiber. 

5 — The  temperature  at  which  the  bleaching  is  car- 
ried out,  chiefly  with  a  view  to  accelerate  the  chemical 
action  between  the  chlorine  and  the  coloring  matter. 

6 — The  time  allowed  for  bleaching,  controlled  by  the 
temperature  and  density  of  the  stock  under  treatment. 

With  regard  to  these  factors,  it  may  be  stated  in 
general  terms,  first  that  it  has  been  found  in  manu- 
facturing practice,  the  greater  the  yield  of  fiber  from 
unit  weight  of  raw  fibrous  plant,  no  matter  by  what 
process  this  fiber  has  been  made,  the  greater  is  the 
loss  of  weight  of  pulp  operated  upon,  and  the  amount 
of  bleaching  agent  required ;  and,  second,  that  the  dilu- 
tion of  the  fiber  with  water,  or  density  of  the  stock, 
and  the  temperature  employed  for  bleaching  are  most 
important  since  the  first  manifestly  results  in  great 
economy  of  steam  and  the  other  in  economy  of  bleach. 

The  oxidizing  material  universally  employed  in  the 
pulp  industry  is  chlorine  in  the  form  of  bleaching  pow- 
der (calcium  hypochlorite)  or  an  electrolyzed  solution 
of  common  salt,  for  chlorine  is  unquestionably  the 
cheapest  bleaching  agent  known.  Ozone  has  been  ap- 
plied, but  failed  to  compete  with  chlorine  on  account 
of  its  cost;  and  although  permanganates  may  in  spe- 
cial cases  be  used  to  obtain  a  greater  degree  of  white- 
ness than  can  be  obtained  with  hypochlorites  alone, 
yet,  the  use  of  a  permanganate  is  scarcely  admissible 
in  the  pulp  and  paper  industry  on  account  of  the  diffi- 
culty of  removing  the  last  traces  of  manganese  from 
the  fiber.  Hydrogen  peroxide  is  out  of  the  question  at 
the  moment,  owing  to  its  cost. 

In  the  older  methods  of  bleaching,  the  hollander 
with  a  paddle  wheel  to  throw  the  pulp  over  the  back- 
fall instead  of  a  roll,  and  sometimes  called  a  "pocher," 
was  used,  the  fiber  being  allowed  to  circulate  either 
cold  or  hot  until  the  required  degree  of  whiteness  was 
attained.  The  stock  at  first  was  washed  in  the  hol- 
lander after  bleaching,  with  drum  washers,  the  wash- 
ings containing  the  excess  bleach  being  thrown  away. 
Or,  instead  of  washing,  a  quantity  of  sodium  hypo- 
sulphite was  added  to  destroy  the  excess  of  hypo- 
chlorite present.  This  was  obviously  a  very  wasteful 
method  and  a  distinct  improvement  was  the  intro- 
duction of  a  special  draining  tank,  usually  built  of  con- 


crete  and  provided  with  a  perforated  false  bottom  of 
earthenware  tiles,  into  which  the  pulp  was  emptied  and 
drained,  the  liquor  being  pumped  back  again  to  the 
"pocher"  to  be  mixed  with  a  quantity  of  fresh  un- 
bleached pulp,  so  as  to  exhaust  any  available  chlorine 
it  contained.  These  draining  tanks  are  in  use  today 
in  many  papermills. 

A  still  further  advance  was  made  when  a  series  of 


open  concrete  tanks  were  put  down  to  bleach  and  store 
large  quantities  of  pulp,  the  main  principle  being  to 
thoroughly  mix  the  bleach  liquor  and  pulp  together 
in  a  suitable  hollander  or  "pocher,"  and  after  steaming 
to  the  required  temperature,  running  the  whole  charge 
into  a  tank,  there  to  remain  till  the  fiber  came  up  to 
the  requisite  degree  of  whiteness,  a  slight  excess  of 
bleach  liquor  being  added  for  this  purpose.  The  liquor 
was  then  drained  off  into  a  well  and  from  there  pump- 
ed back  to  the  "pocher"  to  meet  fresh  unbleached  pulp, 
thus  becoming  exhausted  of  its  available  chlorine. 
The  density  of  the  stock  was  in  all  cases,  attained 
with  the  drum  washer. 

An  arrangement  of  the  kind  described  above  is 
shown  in  Fig.  I.  The  fiber  direct  from  the  screens 
flows  into  the  pocher  a,  and  after  the  desired  density 
has  been  obtained  with  the  drum  washer,  the  bleach 
liquor  is  added  and  the  whole  circulated  for  a  couple 
of  hours  or  so,  steam  being  injected  meanwhile  till 
the  temperature  reaches  100  to  no  to  120°  Fahr. 
The  charge  is  then  run  off  through  the  chute  b,  into 
any  one  of  the  series  of  tanks  c,  where  it  remains  at 
rest  for  twelve  or  sixteen  hours.  By  this  time  the 
pulp  will  have  reached  a  good  color.  The  liquor  is 
then  drained  off  through  the  plug  hole  d,  into  the  pipe 


e,  which  conveys  it  to  the  well  f,  from  whence  it  is 
pumped  back  to  the  pocher  again.  The  drained  and 
bleached  fiber  is  afterward  conveyed  to  the  beater 
floor  in  trucks,  or  thrown  on  the  traveling  belt  g,  and 
conveyed  to  the  stuff  chest  h,  mixed  with  water  and 
pumped  partly  to  a  wet  machine  on  the  beater  floor, 
to  be  made  into  laps,  and  partly  direct  into  the  beating 
engines.  Such  a  system  manifestly  involves  much 
labor,  plant  and  floor  space,  not  to  mention  a  somewhat 
large  expenditure  of  steam  for  heating,  when  hot 
bleaching  is  carried  on.  As  a  rule  not  more  than  2.5 


to  3  percent  density  of  stock  is  obtained  from  the 
pocher. 

Bleaching  apparatus  were  then  designed  fulfilling 
more  perfectly  the  conditions  for  economy  .  The  ves- 
sels or  pochers  were  built  of  meunier  work,  or  rein- 
forced concrete,  and  the  mixture  of  pulp,  bleach  liquor 
and  water,  kept  in  continued  motion  by  means  of  an 
Archimedean  screw  or  propeller,  until  the  process  of 
bleaching  was  practically  completed.  Such  a  system 
is  illustrated  in  Fig.  2. 

The  propeller  is  placed  at  one  end,  and  in  this  par- 
ticular case  causes  the  stock  to  travel  in  the  direction 

5 


of  the  arrows.  These  bleaching  engines  are  built  to 
hold  from  one  to  ten  tons  of  air-dry  pulp  per  charge, 
and  are  operated  with  from  four  to  six  percent  stock. 
Such  a  degree  of  concentration  insures  fair  economy 
in  bleach  and  steam,  but  they  are  intermittent  in  their 
action,  and,  in  consequence,  there  is  considerable  loss 
of  time  in  filling  and  emptying.  The  stock,  after  it 
has  acquired  the  right  degree  of  whiteness,  is  emptied 


into  the  chest  beneath,  from  whence  it  is  pumped  to 
the  washing  and  drying  machines.  The  names  of 
Kellner,  Partington,  Bdmer,  Hromadnik,  and  others 
are  identified  with  bleaching  engines  and  systems  of 
this  kind. 

It  has  always  been  the  aim  of  pulp  manufacturers 
to  invent  a  continuous  system,  or  one  that  is  nearly 
so,  and  quite  a  number  of  such  have  been  constructed 
and  operated  for  many  years  in  Europe  and  America. 
Such  plants  are  known  in  Great  Britain  as  the  "tower 
system"  and  in  America  as  the  "continuous  tank  sys- 
tem." Neither  of  these  fulfills  the  most  perfect^  con- 
ditions for  bleaching,  although  the  tower  system  is  the 
better  of  the  two.  The  towers  are  usually  concrete 
tanks,  with  or  without  conical  bottoms,  connected  to- 
gether by  channels  or  passages  as  shown  in  ^  Fig.  3, 
which  illustrates  the  "continuous  tank  system"  in  use 
in  many  mills  in  America.  This  consists  of  six  or 
more  circular  concrete  tanks  12  feet  in  diameter  by 
about  20  feet  deep,  connected  together  with  passages 
or  pipes,  at  top  and  bottom  alternately,  as  shown  in 
the  section  through  A  B.  These  tanks  have  fiat  bot- 
toms and  agitators  driven  by  spur  gearing  at  the  top, 
which  keeps  the  pulp  in  continuous  motion.  The  pipes 
or  passages  connecting  the  tanks  at  the  bottom,  are  all 

6 


on  the  same  level,  but  those  for  the  overflow  from  2 
to  3,  4  to  5,  6  to  7,  and  so  on  through  the  series,  are 
all  on  different  descending  levels,  in  order  to  permit 
the  pulp  to  flow  by  gravity  from  the  first  to  the  last 
tank  in  the  series  in  the  direction  as  shown  by  the 
arrows. 

Instead  of  the  stock  flowing  by  gravity,-  it  is  some- 
times pumped  from  one  tower  to  the  next  in  series. 
This  obviously  is  forced  circulation,  and  has  certain 
advantages  over  circulation  by  gravity,  but  can  scarce- 
ly be  called  a  continuous  system.  It  permits  o*f  great- 
er concentration  of  stock,  a  stronger  bleaching  fluid 
in  intimate  contact  with  the  fiber,  and  economy  of 
steam  for  heating  when  hot  bleaching  is  employed, 
but  under  the  best  conditions,  seldom  more  than  4 
to  4.5  percent  stock  can  be  handled,  which  in  the 
opinion  of  many  is  too  dilute  to  yield  the  most  econ- 
omical results  in  any  continuous  system. 

Skjold  (Svensk  Pappers-Tidning,  1905,  No.  15, 
pp.  85-86)  shows  a  method  of  continuous  bleaching 
in  which  he  employs  a  series  of  flat-bottomed  upright 
circular  tanks,  built  of  concrete  and  containing  agita- 
tors, four  or  more  tanks  being  employed  in  the  series, 
connected  by  an  arrangement  of  pipes,  so  that  the 
pulp  can  be  pumped  from  one  to  the  other  continu- 
ously, or  circulated  at  will  from  the  bottom  to  the  top 
of  each  tank;  a  mixing  tank  is  provided  between  the 
wet  machine  and  the  first  bleaching  tank,  for  mixing 
the  sheet  of  wet  pulp  with  water  of  50° 'Cent.  (122° 
Fahr.)  and  bleaching  powder  solution  of  3.5°  Be. 
The  bleached  pulp  leaving  the  system  is  washed  on 
wet  machines  before  it  is  dried.  The  tanks  in  this 
system  are  each  4.9  meters  in  diameter  by  8  meters 
deep,  giving  a  total  capacity  for  the  four,  of  about 
600  cubic  meters,  and  it  is  stated,  that  from  forty  to 
forty-five  tons  of  air-dry  pulp  can  be  bleached  in 
twenty-four  hours,  equivalent  to  nearly  500  cubic  feet 
of  tank  space  a  ton  a  day.  This  is  a  large  output, 
which  is  made  possible,  perhaps,  by  the  high  tempera- 
ture employed — viz. :  f rom  130  to  140°  Fahr.  The 
operations  of  this  system  are  somewhat  broken,  and 
it  is  doubtful  if  these  conditions  as  to  output  could  be 
maintained  in  constant  practice,  unless  the  stock  treat- 
ed were  of  the  easiest  bleaching  character. 

J.  E.  Heiskanen,  in  his  apparatus  (U.  S.  Pat. 
1277926),  has  overcome  certain  difficulties  and  has 
greatly  simplified  the  continuous  system.  His  appa- 
ratus is  illustrated  in  Fig.  4,  5  and  6.  All  stock  pipes, 
centrifugal  pumps  and  agitators,  are  eliminated,  the 

7 


inventor  substituting  for  these  propellers  for  mixing, 
agitating  and  circulating  the  stock  through  the  whole 
system.  These  propellers  are  driven  by  small  motors 
fixed  on  the  upright  shafts,  or  by  overhead  gearing, 
and  as  he  attains  a  density  of  6  to  8  percent  stock  he 
fulfills  4:he  best  conditions  for  economy  of  bleach 
liquor  and  economy  of  steam  for  heating.  The  floor 
space  occupied  by  the  plant  is  about  half  of  that  re- 
quired for  the  "continuous  tank  system"  mentioned 
above. 

The  unbleached  pulp  falls  from  the  pulp  thickener 
into  the  screw  conveyor  where  it  is  mixed  with  the 
necessary  quantity  of  bleach  liquor  and  hot  water,  the 
mixed  stock  being  conveyed  automatically  into  the 
first  bleaching  tank  la.  The  fiber  in  this  tank  is  forced 
upwards  by  the  propeller  to  the  top  of  Ib,  where  the 
stream  is  divided  by  the  regulating  gates  into  two 
{Jarts,  one  part  giving  back  into  la  while  the  other 
part  flows  into  I  la.  The  proportion  going  forward 
into  Ha  varies  from  one-tenth  to  one-fifth  of  the  total 
volume  passing  the  propeller,  so  that  the  stock  in  la  is 
kept  circulating  vigorously  and  is  in  continual  motion. 
These  regulating  gates  are  placed  at  the  top  of  Ib, 
lib,  I  lib,  and  so  on  through  the  whole  series,  the  flow 
forward  from  one  tank  to  another  being  adjusted  by 
them  in  accordance  with  the  amount  required,  and 
kind  of  pulp  to  be  bleached.  Steam  is  introduced  at 
the  -bottom  of  each  tank  immediately  below  the  pro- 
peller as  shown  in  Fig.  5,  to  maintain  a  uniform  tem- 
perature throughout  the  apparatus.  After  the  pulp 
Has  traversed  through  the  series  of  tanks  it  is  dis- 
charged into  the  chest  at  the  end,  from  whence  •  it 
flows  by  gravity,  or  it  is  pumped,  to  washers  and 
finally  to  the  drying  machine.  The  return  pump  shown 
in  Fig.  4  is  not  essential  and  is  seldom  or  never  used, 
but  is  added  to  enable  the  operator  to  pump  the  stock 
a  number  of  tanks  back,  if  by  accident  insufficient 
Bleach  liquor  has  been  added. 

Heiskanen  represents  that  his  plant  occupies  half 
the  floor  space  of  the  ordinary  "continuous  tank  sys- 
tem" ;  that  he  can  attain  a  density  of  7  percent  stock 
on  an  average;  and  that  by  so  doing  the  consumption 
of  steam  ancTBleach  is  reduced  to  a  minimum.  The 
consumption  of  power  and  labor  is  extremely  low; 
for  in  the  first  case  there  is  very  little  weight  to  be 
moved  by  the  propellers,  the  columns  of  stock  in  la 
and  Ib  equalizing  themselves,  while  in  the  second  one 
man  is  sufficient  to  run  the  tanks  from  the  first  tank 
to  the  pulp  chest.  He  also  allows  eight  hours  or  so 


II 


for  bleaching  and   completes   this   with   a   total   tank 
capacity  of  about  200  cubic  feet  a  ton  of  pulp  a  day. 

The  plant  as  shown  is  capable  of  handling  100  tons 
of  pulp  a  day,  is  well  designed  and  as  it  is  constructed 
of  concrete,  lined  internally  with  glazed  tiles  if  so  de- 
sired, and  fitted  with  bronze  working  parts  in  con- 
tact with  the  fiber,  the  risk  of  iron  spots  appearing  in 
the  dried  pulp  is  avoided. 

From  the  foregoing  it  is  apparent  that  any  econ- 
omical system  of  bleaching  must  involve  a  high  den- 
sity of  stock,  primarily  to  economize  steam  for  heat- 
ing, and  as  this  is  a  very  important  factor,  I  append 
my  formula  for  calculating  the  amount  required, 
which  is  applicable  to  every  case  for  hot  bleaching: 
(W  S  -\-  w'  s'  -f  w"  s"  + )  (Pf  —  fa')  _ 

~T~  T-tf 

in  which 

S  =  It)  of  steam  required. 

W  =  Wt.  of  air-dry  pulp  in  the  charge,  in  lb. 

s=Sp.  heat  of  air-dry  pulp   (0.65). 

w'  —  Wt.  of  water  associated  with  the  pulp,  in  lb 

s'  =  Sp.  heat  of  water  (i.oo). 

w"  —  Wt.  of  vessel  in  which  pulp  is  bleached,  in  ft>. 

s"=Sp.  heat  of  material  of  which  w"  is  constructed. 

ti  =  Initial  temperature  of  stock  in  degrees  Fahr. 

tf  =  Final  temperature  of  stock  in  degrees  Fahr. 

T  =•  Total  B.  thermal  units  in  lit)  of  steam  used  for  heating. 

From  this  formula  the  following  quantities  of  steam 
required  for  different  densities  of  stock  bleached  at 
different  temperatures  have  been  calculated,  taking 
the  initial  temperature  t,  as  60°  Fahr. ;  the  final  tem- 
perature tf,  as  90,  100,  no  and  120°  Fahr.,  and  the 
total  British  thermal  units  in  one  pound  of  steam  T  as 
1190,  i.  e.,  steam  at  notb  pressure  above  atmosphere. 


Density  of 
Stock 
Pulp     Water 

Water 
associated 
with  20Oott> 
air-dry 
pulp 

Lbs.  of  steam  required 
the  mixture  containing 
air-dry  pulp  to 

90°  F.      100°  F.      110°  F. 

to  heat 

2OOOft) 
120°  F. 

37 
.!  ' 
5* 

D  ' 

7] 

8  ' 

o          97% 
96" 
95" 
94" 
93" 
92" 

646661b 
48000  " 
38000" 
31333  " 
26571  " 
23000" 

1881 
1427 

H54 
972 

842 
745 

2421 
1809 
1442 
1197 
1023 
892 

3053 
2282 
1819 
1510 
1290 
1125 

3699 
2764 
2203 
1830 
1562 
1269 

NOTE.— This  table  is  based  on  the  above  formula  but  the 
weight  of  the  apparatus  w"  has  been  eliminated.  Three  per- 
cent should  be  added  to  "the  above  quantities  of  steam  in  cols. 
4,  5,  6  and  7  to  allow  for  loss  of  heat  by  radiation. 

12 


5160 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


